This invention is directed to an electronic detent apparatus and method for simulating the effect of a mechanical detent. The use of mechanical detents to position mechanical apparatus is well known in the art and are used in a variety of applications, including in conjunction with positioning radiographic equipment.
In radiographic procedures it is frequently necessary for an operator to manually position an x-ray assembly (i.e., a movable axis or movable assembly) to a previously specified precise position or configuration. For example, it is common to position the focal spot of the x-ray device about 1 meter above the receptor plane, with the focal spot centered front to rear and side to side relative to the receptor plane and with the centerline of the collimating device aligned with a line from the center of the receptor to the focal spot. To do this, mechanical locating features have been previously used to provide the operator with an indication that he is approaching the previously specified location. These prior mechanical locating features hold the assembly or movable axis in the specified location and return the assembly or movable axis to the specified location if moved a small distance. These mechanical locating features were and are commonly referred to as detents. The assembly or movable axis can be fixed at multiple pre-specified locations through the use of multiple mechanical detents.
Past mechanical detents came in many variations. The most common designs included the use of a spring-loaded roller (or plunger) which operated on a surface having grooves or recesses. Movement of the movable axis caused relative motion between the spring-loaded roller and the surface, with the spring-loaded roller engaging the groove or recess at a pre-specified position. Engaging the groove or recess brought the movable axis to a stop at the pre-specified location within the groove or recess. As the spring-loaded roller engaged the groove or recess, the operator experienced a xe2x80x9cpullxe2x80x9d as the roller accelerated into the groove or recess. The feel of being pulled into the detent provided the operator with tactile feedback as to whether the detent had been reached. This type of mechanical detent design is common to many other types of equipment, including automotive gear shift levers.
Alternatively, prior detent designs could use magnets physically attached to a rail. A circuit board containing Hall effect sensors would activate an electronic switch to bring the movable assembly to a stop when the circuit board was passed by the magnet(s) physically attached to the rail. The magnets in this alternative prior mechanical detent design were physically affixed to the rail in the same manner as the previously discussed mechanical ramps.
The disadvantages associated with these prior mechanical detents were numerous. For example, adjusting the detenting force was difficult because the detent was a physical object (e.g., a steel wedge), or had a fixed property (e.g., a fixed magnetic field) that was difficult to change. Adjusting or changing the detenting positions was difficult because the mechanical detent had to be physically moved. Adding or removing mechanical detenting positions was very difficult. Also, mechanical detents were unreliable, wore out, and cost money. Finally, it was often difficult to find enough space for multiple mechanical detents, which resulted in additional design time as well as designs that were bulkier than they would be otherwise.
Therefore, a need has long existed for a new and improved detent apparatus and method that overcomes the difficulties associated with past mechanical detents.
In a preferred embodiment of the invention, the electronic detent apparatus includes a sensor connected to a microprocessor. A servo-motor is connected to the microprocessor and has a motor drive connected to a clutch. The clutch may engage a wheel disposed upon a rail or surface to effect the simulation of a mechanical detent through the microprocessor controlled servo-motor.
In a preferred embodiment of the invention, a method for simulating a mechanical detent comprises the steps of moving an axis and monitoring the position and velocity of the axis. The position and velocity of the axis is then compared to a pre-specified position threshold value and a pre-specified velocity threshold value using a microprocessor disposed on the axis. A servo-motor is activated to accelerate the axis to a pre-specified position using a clutch controlled by the servo-motor when the position and velocity of the axis exceed the pre-specified position and velocity threshold values. The servo-motor drive speed is adjusted to nearly match the speed of the axis before the clutch is engaged. The clutch operates on a wheel connected to the clutch and disposed upon a rail. In an alternative embodiment of the invention, the step of monitoring the position and velocity of the axis may be limited to monitoring only the position or velocity of the axis depending on the desired control characteristics.
The preferred embodiment of the electronic detent apparatus and method has a number of advantages. First, the number of detents can be easily changed by adding pre-specified position and/or velocity threshold values to the microprocessor. Second, the location of the detent(s) can be easily changed by altering the pre-specified position and/or velocity threshold values. Third, because the detent(s) are electronic they occupy negligible space, unlike mechanical detents, thereby reducing detent device size and design times. Fourth, the method of the present invention provides great flexibility in positioning and changing the number of detents. Sixth, the use of an electronic detent allows for numerous detent characteristics (such as, for example, detent length and xe2x80x9cpull forcexe2x80x9d experienced) to be varied in a virtually limitless number of ways, e.g., either in a pre-specified manner or continuously. Other features and advantages of the invention will become apparent from the description that follows.